DC-to-DC voltage regulators generally regulate an input DC voltage to produce an output DC voltage with a stable DC level. Examples of DC-to-DC voltage regulators include linear regulators, switching regulators, etc. The impedance of a linear regulator generally varies with the load on the regulator, resulting in a substantially constant output voltage. Switching regulators generally perform DC-to-DC voltage conversion and regulate the converted DC voltage to produce an output voltage with a stable DC level. DC-to-DC voltage conversion involves converting a DC input voltage into an output voltage with a different DC level. The DC-to-DC voltage conversion circuit of a switching DC-to-DC voltage regulator generally includes a plurality of switching devices, e.g., one or more field effect transistors (FETs), or, in the alternate, one or more FETs in combination with one or more diodes. The switching devices generally operate to provide a signal (e.g., a modulated square wave signal) that can swing from a first rail, e.g., a high supply, VH, to a second rail, e.g., a low supply (ground), VL, or to a voltage in between. The voltage regulation circuit of a switching DC-to-DC voltage regulator generally includes a filter. The filter provides a DC or substantially DC output voltage by smoothing the signal provided by the switching device(s).
FIG. 1 shows an example of a DC-to-DC voltage regulator 10 that produces an output voltage (VOUT) that is less than the input voltage (VIN). The DC-to-DC voltage regulator 10 of FIG. 1 is sometimes referred to as a synchronous buck converter. The regulator 10 includes a first switching device 12 and a second switching device 14 that are controlled by a controller 15, e.g., a device capable of controlling the switching devices 12, 14 utilizing pulse width modulation (PWM), pulse frequency modulation (PFM), or other suitable techniques. When only the first switching device 12 is activated or ON, an input voltage (VIN) is introduced at node 11, and when only the second switching device 14, which is grounded, is activated or ON, there is zero voltage at node 11. The duty ratio (D) of the regulator 10 depends on the ON time (T1) of the first switching device 12 during a cycle and the ON time (T2) of the second switching device 14 during a cycle, and is given by the equation:D=T1/(T1+T2).
A filter including, for example, an inductor(s) 16 (L) and output capacitor(s) 18 (COUT) are provided downstream of the node 11, to filter out the signal generated at node 11 and thereby produce a substantially DC output voltage (VOUT).
Conventionally, according to the prior art, a DC-to-DC voltage regulator could be divided amongst a number of integrated circuits. For example, the controller 15 could be on one chip or die, each power FET 12, 14 could be on a separate chip or die or on separate oxide layers on the same chip, and another chip or die might have been provided to drive the FETs 12, 14. Alternatively, all of the components could be integrated on a single chip.
Multi-stage or cascading DC-to-DC voltage regulators can be implemented using circuits in which the output voltage of a first voltage regulator becomes the input voltage of a second voltage regulator. An illustrative two-stage regulator 20 is shown in FIG. 2. In the example of FIG. 2, the two-stage voltage regulator 20 includes a first voltage regulator stage 22 and a second voltage regulator stage 24. The first voltage regulator stage 22 provides a first stage output voltage 21, and the output voltage 21 of the first voltage regulator stage 22 is coupled to the input of the second voltage regulator stage 24. The second voltage regulator stage regulates its input voltage 23 to provide a second stage output voltage 29. In the example of FIG. 2, the second stage input voltage 23 is equal to the first stage output voltage 21, and the second stage output voltage 29 is also the output voltage of the two-stage voltage regulator 20.